This invention relates to an improved ground mineralisation rejecting metal detector and in particular to an arrangement to assist in power saving.
In U.S. Pat. No. 5,576,624 I disclosed a metal detector apparatus which is capable of substantially reducing signals arising from ground mineralisation containing magnetic soils. Several models utilizing these principles have been commercialised and used highly successfully by gold prospectors and for land mine detection. Both land-mine de-miners and prospectors desire low power consumption so that battery weight and costs are low and operating life is extended. The aim of this invention is to improve upon power consumption.
In U.S. Pat. No. 5,576,624 I disclose a metal detector apparatus which transmits a pulse induction waveform. The advantages of pulse sequence containing pulses of different duration are disclosed. The received signal response from magnetic soils is approximately the same for all such soils (except for the overall amplitude). In contrast, the response from metal targets vary substantially. Hence if a proportion of one part of the received signal is subtracted from a linear combination of a different part of the received signal, It is possible to select the coefficients of the said linear combination such that the response from magnetic soils are cancelled. In general, a particular said linear combination resulting in magnetic soil signal cancellation, a first linear combination, will not result in cancellation of most metal targets, except for some particular metal targets with a particular eddy current time constant decay, a first eddy current decay time constant, which happens to also result in a zero first linear combination result. In order to detect targets with the said first eddy current decay time constant, a different linear combination of different sampled receive signals, a second linear combination, is required to cancel magnetic soil response and not cancel the said first eddy current time constant decay signal. Likewise the said second linear combination will cancel a particular metal eddy current decay signal, a second eddy current time constant decay, which will not be cancelled by the said first linear combination if chosen appropriately. Thus by simultaneously measuring and calculating at least two different linear combinations, all metal targets may be detected and magnetic soil signals approximately cancelled.
Also in U.S. Pat. No. 5,576,624 I disclosed a metal detector apparatus which is insensitive to induced e.m.f. signals from movement of a search coil relative to the earth""s magnetic field and magnetic fields from magnetic soils and rocks. This is achieved by selecting the total integrated receive signal over a complete cycle to be zero.
An object of this invention is to provide improvements to apparatus of the foregoing type where there can be effected some power saving improvement.
In one form of this invention there is proposed a metal detector apparatus including a transmit coil adapted to transmit a magnetic field when operating with electrical current flowing through the said transmit coil, transmit timing control circuitry which also includes timing for a first semiconductor switch, and receive signal circuitry and receive signal processing circuitry;
wherein a damping network including at least one resistor is connected in series with the said first semiconductor switch across the transmit coils and the timing control circuitry generates a timing sequence which is selected so that for at least some of the time no transmit field is generated and at least during part of the transmit signal, the first semiconductor switch is controlled to be off and is turned on before the cessation of the transmit field.
In preference, in a further form of this invention this can be said to reside in a metal detector apparatus including a transmit coil adapted to transmit a magnetic field when operating with electrical current flowing through the said transmit coil, at least one power supply, a first power supply, and transmit timing control circuitry which also includes timing circuitry for a switch mode power supply, and receive signal circuitry and receive signal processing circuitry;
wherein the transmit coil is controlled so that a back e.m.f. following the transmit coil current energising derived from power from the said first power supply, causes current to flow to a capacitor, a first capacitor,
and the said switch mode power is adapted to transfer energy from the first capacitor to another power supply which may be the said first power supply.
In a further form of the invention it can be said to reside in a metal detector apparatus including a transmit coil adapted to transmit a magnetic field when operating with electrical current flowing through the said transmit coil, at least one power supply, a first power supply, and transmit timing control circuitry which also includes timing circuitry for a switch mode power supply, and receive signal circuitry and receive signal processing circuitry;
wherein a transmit coil damping network including at least one resistor is connected in series with the said first semiconductor switch across the transmit coil, and the timing control circuitry generates a timing sequence which is selected so that for at least some of the time no transmit field is generated and at least during part of the transmit signal, the first semiconductor switch is controlled to be off and is turned on before the cessation of the transmit field,
whereby the transmit coil is controlled so that a back e.m.f. following the transmit coil current energising period derived from energy from the said first power supply, causes current to flow to a capacitor, a first capacitor,
and the said switch mode power is adapted to transfer energy from the first capacitor to the said third power supply.
in preference, the switch mode power supply is adapted to operate during transmission and not reception.
In preference, the timing control to the switch mode power supply is adapted by means of a negative feedback servo loop to maintain the average voltage across the first capacitor to a selected value when the apparatus is operating.
Most pulse induction metal detectors transmit a mono-polar transmit signal generated by applying a voltage to a transmit coil, which may also be used for receiving, of typically negative several volts for typically several tens or hundreds of microseconds, and at the cessation of this applied voltage, the resulting back e.m.f. is typically clamped to a few hundred volts and the energy thus dissipated as heat.